The present invention relates to the development of inexpensive materials and means to apply them which are eminently suitable for the correction of iron deficiency-induced chlorosis in plants. More particularly, the present invention relates to the development of certain combinations of a by-product of the citrus fruit processing industry and of iron-containing fertilizer or fertilizer amendment materials (FAM) having characteristics which significantly increase their efficiency to correct iron deficiency-induced chlorosis in plants. The term citrus by-product, as used herein, means the waste or remains resulting after the processing of various whole citrus fruits for juice, feed-stuffs, flavorings, etc., a substantial portion of which is represented by pulp, peel, and seeds, and oftentimes found in combination with minor amounts of stems and leaves. Still more particularly, the instant invention relates to the discovery that certain organic acids, particularly citric acid, contained in the citrus by-product, when wetted by water in the soil, will react to complex and otherwise protect selected iron compounds to provide an economical and readily available iron source imminently suitable for correcting iron deficiencies in plant life growing at such situs. Even still more particularly, the instant invention relates to the discovery that both such by-product citrus material and FAMs should be very finely ground separately and thereafter mixed together in proper portions, thus to maximize the surface area and reactivity of the contained iron and the certain organic acids, particularly citric acid, to provide maximum availability of contained iron to plants. Finally, the instant invention relates to the discovery that the such combinations of citrus by-product and iron-containing fertilizer or iron source FAMs should be band applied in a continuous intact band at or prior to planting or spot placed in the root zone of growing plants in soil to minimize the contact of these products with the soil so that chemical reactions which adversely affect the availability of these products to plants are minimized.
2. Description of the Prior Art
Iron is an essential element in plant nutrition and generally is classified as a micronutrient. It is known to be involved in the synthesis of chlorophyll which in turn is required for photosynthesis in plants. A deficiency of this micronutrient in growing plants, which can be greatly exaggerated in calcareous type soils, is oftentimes the cause of chlorosis, which is characterized by a yellowing of plant leaves and stems and which results in particularly poor growth.
Currently available practices for alleviating such iron deficiencies in growing plants include the application of synthetic iron chelates to soil or the use of various soluble iron compounds as foliar sprays for direct application to the plants or the use of certain hydrophilic polymer delivery systems. Currently, the least expensive, in terms of up-front per unit cost, water-soluble iron compound in use is iron sulfate, either in its reduced state, e.g., (FeSO.sub.4) or in the ferric state, e.g. [Fe.sub.2 (SO.sub.4).sub.3 ]. However, FeSO.sub.4 or Fe.sub.2 (SO.sub.4).sub.3 should not be applied directly to soil lest either source quickly becomes combined with certain components in the soil to form water-insoluble compounds thereby rendering such iron unavailable to growing plants.
The synthetic chelate FeEDDHA [ferric chelate of ethylenediamine (di-(o-hydroxyphenyl acetate))] has been deemed to be the most effective iron fertilizer for soil application, especially in calcareous soils (Arthur Wallace, A Decade of Synthetic Chelating Agents in Inorganic Plant Nutrition, Edwards Brothers, Inc., Ann Arbor, Mich., 1962). However, the per unit cost of iron in FeEDDRA is quite high, which makes this iron chelate material much too expensive for application to relatively low-value field crops. Another currently available and somewhat less expensive iron chelate material, FeEDTA (monosodium ferric ethylenediamine tetraacetate), has proven to be effective for crops growing in near neutral soils but not in calcareous, high-pH soils wherein most iron deficiencies occur. Another recent discovery also somewhat less costly than chelates are hydrophilic polymer delivery systems (Mortvedt, et al., U.S. Pat. No. 5,221,313, Jun. 22, 1993). However, the application of these is difficult and requires specialized equipment; the polymer in the formulations is also relatively expensive. Nevertheless, the initial per unit cost of iron in iron sulfate is significantly lower than in the chelates or the hydrophilic polymer systems. Accordingly, iron sulfate would be the more economical FAM eminently suitable for field crops if it remained available to growing plants subsequent to its contact or juxtapositioning with the soil situs. Therefore, additives or conditioners which can significantly improve the effectiveness of iron sulfate for the treatment of chlorosis could, in turn result in an economically effective iron source FAM for soil application.
Currently, it is the practice in the trade for iron-containing or iron source FAMs to either be applied to soil separately or to be incorporated with other materials in the processing or blending of fertilizers or to be applied in a hydrophilic gel polymer matrix. The effectiveness of iron source FAMs in maintaining a supply of iron to growing plants depends upon the chemical nature of the iron source materials and/or the soil, as well as their rate and/or frequency of application. Economic considerations regarding the use of iron source FAMs are determined by their costs and rate of application and ease of application relative to the returns attributable to increased yields of the crops to which they are applied. Presently, the most effective iron chelate, FeEDDHA, is so costly that its use is restricted to high-cash value crops such as, for example, apples, grapes, and peaches, or high-cash value ornamental crops such as, for example, rhododendrons, azaleas, and dwarf citrus, while other methods, i.e., hydrophilic polymer delivery systems, are nonetheless still expensive in addition to being difficult to apply and are not as effective as FeEDDHA, while the least costly, on a front-end per unit cost basis, iron source FAMs are ineffective when used in procedures designed to correct iron chlorosis in many lower value crops, such as, for example, corn, grain sorghum and soybean.
From the aforesaid, it should now be abundantly clear that the prior art materials designed as, or intended to be, iron source FAMs are either too costly up front to be economical for use on most field crops or are difficult to apply and require specialized application equipment or although available at relatively low unit cost, are still highly uneconomical to use since they are ineffective in maintaining a supply of available iron to crops growing on iron-deficient soils.
Investigations leading to the making of the invention of Mortvedt, et al. (U.S. Pat. No. 5,221,313, Jun. 22, 1993) have now led to the instant unexpected discovery that the use of citric acid or other similar organic acids such as malic or lactic acid, in the hydrophilic polymer delivery system will cause chemical protection to occur for contained iron in addition to the physical protection provided by the hydrophilic gel matrix, and will cause enhanced iron uptake by plants over the FAM in combination with the hydrophilic polymer. The rationale for this is that citric acid is a complexing agent for iron which forms sufficiently strong compounds with iron as to prevent dissolution of the iron from the compound once in soil. Otherwise, the iron subsequently would combine with certain components in the soil to form water-insoluble compounds which are unavailable to growing plants. The fact that citric acid does in fact form such sufficiently strong compounds is attested to by the numerical value of the formation or stability constants which are log K.degree. =4.4 for FeSO.sub.4 and log K.degree. =11.1 for Fe.sub.2 (SO.sub.4).sub.3 However, it was early on recognized that pure grade citric acid or other organic acids adds significantly to the cost of the original hydrophilic polymer delivery systems. A search for a less expensive source of citric acid led to the unexpected discovery that a by-product of the citrus industry known and referred to in the trade supply as citrus pulp, which contains only from 0.5% to about 1.0% citric acid, if finely ground, can be used as a substitute for pure grade citric acid in hydrophilic polymer delivery systems. Moreover, and more significantly, it was unexpectedly discovered that a combination of such citrus by-product (CBP) when very finely ground and mixed into intimate contact with powdered iron sulfate (the whole being referred to as CBP Mix) and thereafter applied in a band to soil situs, was more effective in enhancing plant growth than either FeEDDHA or the hydrophilic delivery system of Mortvedt, et al., supra, or iron sulfate alone. Moreover, and even more significantly, it has now been discovered that since the principal components of said citrus by-product are primarily lignin, cellulose, and hemicellulose plant substances which decompose in soil at varying rates, that it can now herein be postulated that the CBP will provide not only immediately available iron to crops but also over a longer period of time, most likely up to one year. The rates of decomposition one year after addition to soil are detailed in O. L. Smith, Soil Microbiology: A Model of Decomposition and Nutrient Cycling, CRC Press, Boca Raton, Fla., 1982, and are: lignin--50%/yr; cellulose--75%/yr; and hemicellulose--90%/yr. Thus, it is now herein postulated that the instant invention can be effected to provide a novel method for improving the efficiency of iron uptake by plants in calcareous soils.